I filled up the tank today, to the click on the pump and for the first time the fuel gauge only reads one segment away from full. I drove in to another station and it took about £3 worth before it clicked again but still says the reading is the same and not full.

The ‘click’ depends on a number of things, mostly the pressure of the pump - if it’s quick filling then it will ‘backwash’ sooner than a slow pump.

On Citroen’s if you fill with less than 5L the car does not register this - in theory you could fill from empty to full by putting 4L in, starting the car then putting another 4L in numerous times, have a full tank but nothing registering on the gauge.

I tend to wait for my filler to ‘click’, give it a few seconds then squeeze the trigger about half open until it ‘clicks’ again.